为什么初始化后golang slice是空的？

My question here is why the slice is empty in another func when the slice is global to the file?

Here's a piece of code:

package main

import "fmt"

type Vec3 struct {
    x float32
    y float32
    z float32
}

var a []Vec3

func main() {
  a := make([]Vec3, 0)

  a = append(a, Vec3{2.0, 3.0, 4.0})
  a = append(a, Vec3{3.4, 5.6, 5.4})
  a = append(a, Vec3{6.7, 4.5, 7.8})

  fmt.Printf("%+v
", a)
  doSomethingWithA();
}

func doSomethingWithA() {
  fmt.Printf("%+v
", a)
}

Output:

[{x:2 y:3 z:4} {x:3.4 y:5.6 z:5.4} {x:6.7 y:4.5 z:7.8}]
[]

This is a repl.it link too, if you want to take a look.

Thanks for your kind help.